Coatings of this type can have a wide range of uses and the substrate to which the same is applied can be solid surfaces such as metal, glass, ceramics, semiconductors, flexible surfaces such as paper, textiles and/or polymers and the like and indeed any surface which is capable of supporting and retaining the coating thereon. The coating can be controlled to be either generally repellent to all liquids or specifically repellent of particular liquids to suit particular purposes.
The extent or degree of the liquid repellency is known to be a function of the number of fluorocarbon moieties that can be generated and located with respect to the available surface area and also a function of the surface roughness characteristics. In general, the greater the concentration of fluorocarbon moieties and the greater the degree of surface roughness then the greater the repellent characteristic of the coating.
Conventionally a coating of the type of interest in this patent is applied to the surface of a substrate by any of sputter deposition of material from a polytetrafluorethylene (PTFE) target, exposure to F2 gas or using plasma techniques including exposure to fluorine-containing electrical discharges and/or plasma polymerisation of fluorocarbon monomers.
The known technique most often used is the plasma technique which is recognised as being clean, dry, and generating little waste material compared to the conventional wet chemical methods. A plasma is generated from molecules which are subjected to ionising electrical fields and, when completed, and performed in the presence of the substrate, the ions, radicals and excited molecules in the plasma react directly with the substrate or polymerise in the gas phase and react with growing polymer films on the substrate to form the coating thereon.
As stated, it is also known to improve the repellence of the coating by controlling the surface roughness. One method of increasing the surface roughness is to first apply to the surface of the substrate, an intermediate layer of material which has a surface roughness greater than that of the surface of the substrate. The provision of this intermediate layer is described by the Cassie-Baxter equation where surface roughness causes air to be trapped in a void which prevents the liquid from penetrating the surface hence increasing the repellence characteristic of the coating.
The trapping of the air in voids minimises the contact angle hysteresis and results in the provision of what are known as “super hydrophobic” coatings upon which a liquid drop spontaneously or easily move across the substrate coating even in horizontal or substantially horizontal planes.
The provision of intermediate layers applied to the substrate surface to improve the surface roughness are normally achieved by any or any combination of the following:                Sublimation of aluminium acetylacetonate from a boehmite, titania or silica coating,        Sol-gel deposition of alumina and silica,        Anodic oxidation of aluminium,        Photolithographically etched surfaces.        
All of the above processes include a pre-roughening step followed by a reaction of the fluorine containing coupling agent to impart low surface energy.
The aim of the present invention is to provide a method, apparatus and finished article which represent, respectively, improvements with respect to the repellency of the coating applied thereby and onto the substrate surface. It is also an aim to provide the coating in a manner which has the required repellency, is durable and therefore can be commercially exploited.